1. Field of the Invention
The present invention is directed to a reactive power balancing current limited power supply (RPBCLPS) for driving floating DC loads, such as for LED lighting, battery charging stations, and other applications. More in particular, the invention is directed to a power supply which utilizes capacitive reactance to limit the current being supplied to the load, rather than inductance. Accordingly, the present invention provides improved power supply efficiency, limits surge when first powered, improves reactive power balance of the local premises and/or local grid, improves power quality of the electrical grid, and reduces EMI present on the grid.
2. Description of the Related Art
The advent of smart grids and the related evolution in the interconnectivity of electrical power distribution systems, including both upstream and downstream effects of local premises consumption on the power grid, are beginning to change the way that power distribution systems are engineered. For instance, a certain degree of computerized control and monitoring of power consumption is beginning to be implemented as not only energy consumption patterns evolve, but also as new classes of electricity consuming devices emerge. By way of further example, new technologies are giving rise to electrical battery powering of lawnmowers, vehicles, backup power supplies for household or commercial use, etc., such that large scale localized battery charging is becoming an important design consideration. As another example, light emitting diode (“LED”) technology is making its way into mainstream lighting applications, including residential and commercial buildings, alike, as well as in street lights, stadiums, and other municipal applications. Other examples include a variety of intelligent systems on premises, from security applications to heating and cooling, to household appliances.
With such changes come certain new challenges, particularly in the way devices are powered, which can have various effects on the local premises as well as the broader energy grid. This presently entails a variety of drawbacks that remain with implementation of current technologies, in addition to the various shortcomings associated with the aforementioned newly emerging technologies. As such, there is a need for providing an improved means to power both traditional and newly emerging devices.
Before discussing in more detail the various shortcomings of current power supplies and related devices and systems, it will help to provide several definitions which set the stage for understanding the various needs in the art.
“Floating Loads” are electrical loads that are not attached to a fixed potential (such as ground), but “float” up and down the voltage range as needed. Examples of floating loads are: LEDs, LED Strings (series connected), LED arrays (a group of series connected and parallel connected LEDs), batteries, motors, low voltage halogen lamps, incandescent lamps, LED bulbs, resistive heating elements, to name a few.
“Reactive Power Balancing” refers to a device related phenomenon wherein the device, when powered, improves the power factor of a building or grid by adding capacitive reactance to an inductive load.
“Current Limited” means that the maximum current that can flow in the connected load is limited and defined by the maximum voltage and frequency.
“AC” or Alternating Current refers to an electrical generator's output as alternating positive to negative around a neutral with currents flowing in one direction on the positive cycle and in the opposite direction on the negative cycle. By contrast, “DC” or Direct Current flows in a constant direction (unidirectional flow of electric charge).
In light of the foregoing definitions, one problem in the art involves the use of magnetics (inductors or transformers) by typical AC/DC power supplies to reduce voltages to usable levels for solid state components or batteries. Solid state devices and batteries require DC power and cannot work with AC power directly. As such, high frequency switching circuits are used to perform the voltage reduction. This creates high frequency current harmonics that must be filtered at the power supply. Any unfiltered high frequency current harmonics will pollute the “mains” with what is known as “conducted EMI.” As the conducted EMI propagates along the mains “radiated EMI” is generated. Conducted and radiated EMI create problems by interfering with radio, and other wireless control and communication systems.
Also, by using magnetics, rectifying the AC, and adding storage capacitors in the power supply, the power supply uses “reactive energy” to charge inductors and capacitors (reactive components). This technique deviates from the ideal “resistive” load that power companies prefer to drive. Reactive energy is not used up but is conserved and released back into the power grid. This creates a problem in that reactive energy must still be delivered to charge the reactive components. Regarding power supplies, in particular, the inductive nature of the traditional power supply reduces the overall power factor (“PF”) of the building electrical system unless corrected inside the power supply—which adds cost, inefficiency, and complexity to the power supply (therefore, less “green”). Even though a single power supply can correct its own power factor, it only improves the overall power factor by adding more real power to the system; it does not help balance the inductance and reactance present on the grid. So, at best, it is neutral. In other words, while PFCorrected power supplies may not add to the overall reactive energy problem, they do not help it either.
By way of further background, it is noted that “Power Factor” (or simply “PF”) is a measure of how effectively electrical power is being used. A high PF benefits both the customer and the utility, while a low PF indicates poor utilization of electrical power and adds cost to the delivery of power to the customer which is passed on as higher energy cost. PF is a value from 0 to 1 (or 0 to 100%). Resistive is 1 and pure reactance (inductance or capacitance) is 0. The power company prefers to drive resistive loads because that is the power the customer is charged for in their buildings.
Various types of power are at work to provide us with electrical energy. Here is what each one is doing:
“Working Power” (kW or kilowatts), also called “true” or “real” power, is the portion of power used in all electrical appliances to perform the work of heating, lighting, motion, etc.
“Reactive Power” (kVAR's or kilovolt-amperes-reactive), also known as non-working power, exists in inductive loads to generate and sustain a magnetic field or in a capacitor to sustain an electrical field. Common types of inductive loads are motors, compressors and lighting ballasts.
Homes and businesses have mainly resistive and inductive loads, but few capacitive loads such as some compact fluorescent lamps (“CFLs”) or emergency exit signs. The ratio between these two types of loads becomes important with addition of more inductive equipment which reduces the PF from an ideal value of 1 or 100%.
“Apparent Power” (kVA or kilovolt-amperes) is the vector sum of Working Power to Reactive Power.
Power Factor (PF) is therefore defined as the ratio of working power to apparent power (PF=kW/kVA). The typical value is about 0.8. The utility company charges for both how much power the customer actually uses and for the excess power that the utility was required to supply. Thus, low PF results in higher utility costs. Low PF also results in overheating in wires, circuit boards and motors. It is therefore in the customer's best interest to maintain a PF as close to 100% as is economically feasible.
Turning to a more detailed explanation on the definition and role of “Inductive Reactive Energy”: When a voltage is applied to an inductor the current will charge the magnetic field. The magnetic field and current will rise linearly, but will lag the voltage by 90 degrees. Every electric load that works with magnetic fields (motors, chokes, transformers, inductive heating, arc-welding generators) produces a varying degree of electrical lag, which is called inductance. The line current drawn by an inductive load consists of two components: magnetizing current and power-producing current. The magnetizing current is the current required to sustain the electro-magnetic flux or field strength in the machine. This component of current creates reactive power that does not do useful “work,” but circulates between the generator and the load. It places a heavier drain on the power source, as well as on the power source's distribution system. The real (working) power-producing current is the current that reacts with the magnetic flux to produce the mechanical output of the motor.
Turning to a more detailed explanation on the definition and role of “Capacitive Reactive Energy”: When a voltage is applied to a capacitor the current creates an electrostatic field. The voltage rises as the capacitor charges up its field. The voltage will lag the current by 90 degrees. The current thus “leads” the voltage. If a charged capacitor is connected in parallel to an inductor, the capacitor will discharge into the inductor, creating a magnetic field. When the capacitor is completely discharged, the inductor then discharges its magnetic field, and charges up the capacitor. If there was no resistance in the process inherent in real components, this action would go on forever; a true perpetual motion machine. The capacitance acts as a source of energy to charge the magnetic field. The capacitor sees the inductor as a reactive load. The two components are opposite in nature and complement each other. Reactive currents flowing in a power grid are doing the same thing as the example above. As the AC voltage swings positive through zero and then negative, the reactive elements attached to the grid will charge and discharge on each cycle. This creates a current flow first in one direction and then the opposite direction. Most of the electrical loads on the grid are inductive and so the power company adds capacitance to the grid to locally compensate. The more local the capacitance is to the inductive loads the less energy is lost in the wires and transformers.
It is thus important note that when the power company first turns on, it charges up the reactive power in the grid, and then begins delivering real power to the loads. In an ideal world where wires and transformers would not have resistance, this would happen one time and the reactive energy would continuously flow back and forth on the grid without loss. In the real world the wire and transformer resistances create heat, eventually dissipating the reactive energy unless it is regenerated by the power company.
In light of the above considerations, power companies routinely add thousands of capacitors to the grid to reduce reactive energy losses. Many companies are in the business of evaluating building power factor and designing custom solutions for buildings or other localities. They may add capacitors at individual loads or they may add a bulk capacitor at the service point and actively monitor power factor and switch the capacitors in and out of the circuit as needed. The power companies would like the customers to take care of their power factor problem in this way since it helps them deliver power more effectively and they are free from the expense and maintenance of adding capacitance. They are giving incentives to customers by beginning to charge for poor power factor on a monthly basis. It is obviously better to purchase the equipment, own it and get rid of the monthly charge.
It would therefore be beneficial to provide a power supply solution that a) removes existing inductive lighting and other devices from the building or other locale, which reduces the inductive reactive energy required by the building/location; and which further b) adds local capacitance to the local/building power grid.
One more important figure of merit to note is “Power Quality” which determines the fitness of electrical power to consumer devices. Synchronization of the voltage frequency and phase allows electrical systems to function without significant loss of performance or life. The term “Power Quality” is used to describe electric power that drives an electrical load and the load's ability to function properly. Without the proper power, an electrical device (or load) may malfunction, fail prematurely or not operate at all.
The electric power industry typically comprises electricity generation (AC power), electric power transmission and ultimately electricity distribution to an electricity meter located at the premises of the end user of the electric power. The electricity then moves through the wiring system of the end user until it reaches the load. The complexity of the system to move electric energy from the point of production to the point of consumption combined with variations in weather, generation, demand and other factors provide many opportunities for the quality of the supply of electric power to be compromised.
While “power quality” is a convenient term for many, it is the quality of the voltage—rather than power or current—that is actually described by the term. The voltage becomes distorted away from a pure sinusoid. This distortion happens when loads introduce current harmonics (120, 180, 240, etc.) of the 60 Hz sine wave back into the system. These harmonics will alter the shape of the incoming voltage waveform which other equipment will “see.” Switching power supplies, electronic ballasts, phase dimmers, and on/off equipment are the main contributors of current harmonics. If the offending currents are removed, the voltage waveform will once again be purely sinusoidal.
In light of the foregoing background, it would therefore be beneficial to provide a power supply that uses an alternative technology, instead of inductance, to limit the current being supplied to the load. It would be a further benefit for such a power supply to more efficiently pass currents to LED or other devices/loads (thus, improving power quality in a building or other premises as well).
It would also be advantageous if such a power supply solution could permit removal of existing inductive lighting and other devices from buildings or other locales, so as to reduce the overall inductive reactive energy required by the building/location. It would be another important benefit if such a power supply could also add local capacitance to the local/building power grid.
It would be further beneficial for such a power supply solution to limit surge when first powered, improve reactive power balance of the local premises and/or local grid, improve power quality of the electrical grid, and reduce EMI already present on the grid.